1. Field of the Invention
The present invention relates to a method for maintaining the integrity of a seal-forming sheath, in particular a cementing sheath, positioned around a metal casing for an oil, gas, water, geothermal or analogous well.
2. Description of the Related Art
An oil, water or gas field is usually exploited via a well into which a metal casing has been inserted and held in place by a cement sheath to fill the space or annulus between the casing and the borehole. The cementing operation, i.e. putting the sheath into position consists in injecting a cement slurry into the casing to cause the drilling mud in particular to rise up and be evacuated via the annulus which is then gradually filled with the slurry. After the slurry has set and hardened, a cement sheath is obtained which prevents any fluid communication between the various formations through which the well passes, and which acts as a support for the metal casing.
Well-cementing is an operation that is very difficult because it requires several parameters to be taken into consideration and kept under control. For example, a slurry with too high a density can cause the rock to fracture, while a slurry with too low a density can cause external fluids to intrude. While slurry density is a parameter which is relatively easy to control, this is not true of its rheological properties. Such problems, which are inherent to any well-cementing operation, are well known to the skilled person, and solutions generally consist in adding various additives to the slurry, the selection of which is not always clear and varies from one well to another.
However, even in a situation where this cementing operation is carried out under good conditions to obtain a sheath which seals and supports once the slurry has set and hardened, it is not long before that sheath is subjected to mechanical and/or thermal stresses which can cause the sheath to deteriorate, and this can culminate in well operating conditions being put into doubt.
Such problems linked to sheath deterioration over the lifetime of the well are not novel in themselves and are well known to the skilled person, but up until now no practical approach has been made to attempt to provide a solution to such problems.